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This article originally appeared in substantially different
form in Space and Communications.

THE SPACE SHUTTLE MAIN ENGINES: is
there a future?

by

Donald F. Robertson

Setting aside the Space Shuttle's many operational failings,
the Orbiter's main engines are a technological wonder. Only
superlatives can be used to describe them. They burn with unprecedented
temperatures -- over six thousand degrees Fahrenheit -- while handling
fuel more than four hundred degrees below zero. The pressures and power
densities are fully three times higher than any engine used in the
Apollo project to land people on Earth's moon. In many ways, the Space
Shuttle Main Engines may be the United States' supreme technological
achievement.

Nobody but the Japanese
is even trying to duplicate the feat.
And they have taken twenty years and many failures to achieve a
similar, but non-reusable, engine on a much smaller scale. Since the
trend today is to back away from performance in favor of safety margins
and reliability, and since deep space propulsion is slowly moving away
from chemical rockets toward higher energy systems, it is possible that
the SSMEs will remain the most efficient large chemical rocket engines
ever built.

The SSMEs also are the
most reliable of today's rocket
engines. In 168 engine flights, there have been no critical failures at
all, just one early shut-down in flight caused by a sensor problem, and
only four shut-downs on the pad, according to Rocketdyne. Challenger
was destroyed by a Solid Rocket Booster leak. This main engine
reliability has been achieved despite engines being re-use as many as
fifteen times.

Including test failures, these figures equate to a total
reliability of 0.9991 in over 500,000 seconds of operation, according
to Dick McMillion, Rocketdyne's Director of Advanced Chemical
Propulsion. That reliability, and the engine's operating experience,
"has not been approached by any other rocket engine except the [Apollo]
F-1," said McMillion. The SSMEs have twice the total experience of all
the F-1 Saturn-V first stage engines ever fired in the Apollo project.
The F-1 had a reliability of .998 with 250,000 seconds of operation.

None of this means that the SSMEs have no faults. Individual
SSMEs are expensive, costing about $40 million each, according to
Rocketdyne. This is only slightly less than the entire cost of a Delta
launch. Their very complexity makes them delicate creatures, requiring
crews of costly engineers to tinker them into reliable operation.
Report after report has argued that operating the SSMEs remains a risky
proposition; their high reliability record may reflect more on
Rocketdyne than on the engines themselves. NASA considers them one of
the most likely routes to another catastrophic Space Shuttle failure.

Nonetheless, Rocketdyne officials believe that the SSME's
very high thrust-to-weight ratio makes these already-existing engines
ideal for Single-Stage-to-Orbit applications. Boeing has chosen the
engine for their Space Lifter stage-and-a-half design for the Air
Force.

By burning non-stop from the ground almost into orbit,
McMillion said, "The SSME has been flying that Single-Stage-to-
Orbit type mission all its life, since 1981. Being a fully reusable
engine, and having the highest performance of any engine in the world,
the SSME has all the attributes necessary for an SSTO engine. The thing
that has changed over the years is that the aircraft structures, now,
have become quite a bit lighter, which enables an SSTO-type vehicle"
utilizing the Space Shuttle Main Engine's performance.

Rocketdyne also argues that it would take a long and
expensive development program to duplicate the SSME's already-
existing capabilities. "While there are other concepts for SSTO
propulsion systems, they are only on paper," according to Terry Murphy,
Rockwell's Program Manager for Advanced Propulsion Systems. "It takes a
long time to get to a system that is this mature, one that you would be
willing to put onto" SSTO vehicles.

Asked about the European Ariane-V engine, Murphy said, "The
Vulcan is a good engine, but, when you start getting up into
[Single-Stage-to-Orbit] types of missions, specific impulse is
everything. The Vulcan just does not have the specific impulse," since
it is not a staged-combustion engine.

Most modern rocket engines pre-burn fuel, then use the
resulting gas to run turbo-pumps. The turbo-pumps, in turn, pump the
fuel and oxidizer into the combustion chamber. Most engines then dump
the exhaust from the turbo-pumps overboard. "A staged-
combustion engine exhausts the fuel-rich gas from the turbo-pumps into
the main injector," instead of to the atmosphere, said Murphy. There,
this gas "is burned with additional oxygen before exiting the main
combustion chamber," contributing to the overall thrust. "This results
in a much higher efficiency than if the turbo-pump exhaust is dumped
overboard, as in the Vulcan and most competing high-energy engines."

The only exception is the new Japanese LE-7 engine under test
for their new launch vehicle. This is a staged-combustion engine like
the SSME. However, it is not designed to be re-used, which essentially
eliminates it from the SSTO competition. The LE-7 also has too low a
thrust, according to Murphy, although McDonnell Douglas' DC-X and other
SSTO-demonstrator projects are looking for an engine smaller than the
SSME.

Murphy added, however, "I would say that the Japanese are
next closest" to a working SSTO engine, after the United States. The
Russians have demonstrated a staged-combustion engine, but "it uses a
kerosene-based fuel, which offers far too low a performance for an SSTO
vehicle," said Murphy. "Other engine concepts, such as tri-propellant
[engines], have not progressed beyond the design phase, and remain to
be proven."

An improvement program is slowly addressing some of the Space
Shuttle Main Engine's problems by simplifying and reducing the
part-counts of many of the engine's components. New computerized
controllers are now beginning to fly. Rocketdyne is conducting a
"Block-1" re-design of part of the engine for flight in 1995, according
to Sally Stohler, Rocketdyne's Director of SSME Marketing. Block-1
changes include a new Pratt & Whitney oxidizer turbo-pump, and
changing the three-duct hot gas fuel manifold power head to a two-duct
design.

The hot gas manifold power head tubes transfer fuel-rich
turbine exhaust from the pre-burner to the main combustion chamber.
Computational Fluid Dynamics analysis showed that the two-duct design
reduced pressure gradients within the system, and eliminated pressure
variations, said Murphy. These changes reduced the pressure and
temperature extremes inside the engine.

Block-2 changes, intended to fly in early 1997, include a new
fuel turbo-pump, and widening the thrust chamber throat. The latter
change again reduces the total pressure differentials within the
engine, reducing stress, which should extend engine life. Rocketdyne is
looking beyond these first two block changes, to new low-cost
advanced-technology components still in the conceptual stage, but the
company declined to talk about these ideas in detail.

SSMEs may be the best engines flying, but reducing engine
weight may still prove essential for true Single-Stage-to-Orbit flight.
Stohler said it may be possible to reduce the weight of the nozzle with
new materials. However, she added, "The original challenge of the Space
Shuttle Main Engine was keeping the weight very, very low. Without
introducing more advanced technology or simplifying the engine, it
would be tough to take more weight out." An SSME currently weighs about
7,000 pounds [3,200 kilograms].

Nonetheless, Rocketdyne argues that, if a new large launch
vehicle is developed, it would be cheaper to continue incremental
improvements to an existing engine than to develop something new from
scratch. Asked if there were any limiting factors on how long SSMEs
could be kept flying, McMillion said that upgrades will continue, and
"we are going to be marketing this engine for the foreseeable future."
He expects no problems keeping the vendor base in line.

The Rocketdyne officials declined to comment on whether
Rockwell, the owner of Rocketdyne, is designing a new launch vehicle to
use the SSME. However, a Congressional source critical of the SSME
pointedly told Space and Communications that
they were not.

McDonnell Douglas' DC-X Single-Stage-to-Orbit (SSTO) project
has argued that both the SSME and the new Space Transportation Main
Engine are far too large, even for the operational DC-Y. NASA's STME is
likely to be canceled if Congress declines to fund the Air Force's
Space Lifter project.

For a smaller, demonstration SSTO like the DC-Y, a new rocket
mid-way between Pratt & Whitney's RL-10 and the SSME or STME
would need to be developed, possibly from the old Apollo J-2s. The
Rocketdyne officials agreed, also pointing out that Shuttle engines
will throttle only to sixty-five percent power. A vertically landing
vehicle like the DC-Y design needs engines that can throttle almost to
zero. For large SSTO vehicles that land horizontally on a runway,
Rocketdyne believes the SSMEs remain ideal.

Futuristic Single-Stage-to-Orbit rockets may be beyond the
United States' means, but Rocketdyne is actively marketing the SSME to
many of the companies competing in United States Air Force's Space
Lifter competition. Boeing chose the Shuttle engine for a partially
reusable stage-and-a-half design. Rockwell said that there are other
customers for the engine who wish to remain confidential.

Vince Caluori, Boeing's Space Lifter Program Manager, said
that his company is trying to address three issues with their Space
Lifter design: low development cost, high mission and dispatch
reliability, and low cost per flight. The Space Shuttle Main Engine is
relevant to all three.

The Boeing Space Lifter -- which Caluori called his "Ariane-
killer" -- is a conservative design, generally avoiding new technology.
"This vehicle is straight-forward as can be, we've got an aluminum
tank, no exotica, no fancy materials," said Caluori. It is intended to
place 30,000 pounds [13,600 kilograms] in low Earth orbit or 10,000
pounds [4,500 kilograms] in geostationary orbit

The Boeing rocket is a very conventional stage-and-a-half
design, similar to the Atlas. It is propelled by two recoverable
"engine modules," each containing two engines, mounted at the base of
rocket. Like the Atlas, the engine modules feed from the central fuel
tank, but unlike the Atlas, the central core does not ignite until the
engine modules shut down almost in orbit. A number of different,
specialized cores and upper stages are assembled in different
configurations to vary the vehicle's performance. The cores and upper
stages are expendable.

"Using the same booster, tank module, and engine modules,
widely differing payload ranges are served by changing the expendable
upper stage," said Caluori. "Delta- and Atlas-class payloads would use
a simple low-energy upper stage, while Titan-
class payloads use a cryogenic stage."

In designing their Space Lifter, the first issue Boeing
addressed was low development cost. Caluori said, "We have to deliver a
system that the country can afford, and the country has spoken clearly
that we cannot afford ten or twelve billion dollars." Boeing thinks
they can develop their Space Lifter for under $5 billion. The company
intends to achieve that by avoiding development of a new engine and by
addressing the entire payload range with a single vehicle.

Once the decision was made to go with an existing engine,
Boeing believed there was only one real choice. "The best existing
engine in the whole world, and the only reusable engine, is the SSME,"
said Caluori. Sounding remarkably like the salesman, rather than the
customer, Caluori said, "It has the highest performance, the best
thrust-to-weight. At reasonable power levels it has proven to be
extremely reliable, it is a continuing program, it is throttleable --
it has all the features you would like to have." Caluori admitted,
"There is no question we would like to have a more robust engine," but
he believes Rocketdyne's on-going improvements should address that
issue. In addition, "Engine integration is always the big issue on a
fairly conventional rocket," said Caluori. With SSME's, "I've got a
known factor in the engine," which should reduce development risk.

The second issue involved reliability. Boeing believes that
propulsion lies at the root of most launch vehicle failures. The
company looked at their experience with commercial aircraft -- the
company's principle product. Modern commercial airliners have only two
engines, which means that in an emergency one engine must be able to
power the entire airplane. The engines are vastly over-powered and
"idle along" at well below their rated power during normal operation:
the engines are not stressed.

Applying the same philosophy to a launch vehicle led Boeing
toward very large engines, that is, toward SSMEs. Their vehicle uses
four SSMEs -- two in each booster -- each operating at seventy-five
percent power. This has three advantages: rocket engines operating a
low power are far less likely to fail at all; when they do fail, they
"almost assuredly" will not do so catastrophically since the engines
are operating at relatively low pressure; and after a non-catastrophic
failure the launcher can continue its mission on the three remaining
engines, now operating at one-hundred percent power. "For the first
time," said Caluori, "we are talking about a vehicle that is going to
cruise to space at `cruise power'," resulting in "an incredible
increase in reliability."

The dispatch reliability of an airliner is directly related
to the amount of avionics redundancy. The same is true of a space
launch system. Boeing added four complete strings of avionics to their
Space Lifter so that a failure could be tolerated on the ground without
giving up the flight. This also is based on experience with Boeing's
Inertial Upper Stage. Caluori said that the IUS is the first and only
United States automated rocket with completely redundant avionics, and
he claimed that redundancy has saved at least three missions after the
first avionics string failed.

Most launch delays are caused by last minute launch pad
repairs to the avionics. Having four complete avionics strings allows
most maintenance to be deferred until after a mission has flown and
landed, avoiding work on the launch pad.

The third issue was low cost per flight. That is achieved by
recovering all of the expensive hardware: the SSMEs, the avionics, the
electrical power systems, auxiliary propulsion, and the thrust
vectoring system. These systems are packaged together in very simple,
oval-shaped ballistic recovery modules. The recovery modules fly almost
all the way to orbit, then the ballistic module separates from the core
vehicle using standard explosive bolts. Prior to reentry, each module
is oriented with nitrogen thrusters. Large parachutes open to gently
place the engine modules into the ocean for recovery. An inflatable
"door" covers the engines. Successfully recovered pods are reused "as
is" after minor refurbishment, simply attached to a new core stage.

Rocketdyne's Sally
Stohler said she thinks the Space Shuttle
Main Engines are still capable of the original fifty-five re-
flight goal. Because of its current low flight rate, the Shuttle
program is aiming for only thirty reuses. Boeing is planning only ten
to twenty re-uses for economic reasons.

Asked if end-of-life
engines that have flown too often to be
safe on the human Shuttle might be sold cheap to Boeing or another
customer for automated flight, Stohler said, Yes. But since NASA owns
the engines that would be up to the space agency. Caluori was
unimpressed: "We [Boeing] don't want any old engines since we would not
throw them away." Stohler added that all of the Shuttle flight engines
are still being flown; none are even close to their end-of-life.
Rocketdyne has essentially completed all new engine builds, and no
further engines are planned for the life of the Shuttle program. The
new upgrades will be retrofitted later to the existing engines.

Boeing's bottom line? "We think we can launch for $50 million
dollars per flight," said Caluori. "$50 million gives the reliability
we have, which nobody can match with any of those [Russian or Chinese]
vehicles. It looks very attractive, especially if we double-manifest,"
or carry two satellites at a time, as on Europe's Ariane. Referring to
Ariane, Caluori said, "I think we've got something that will hurt."

Boeing has spent $10 to $15 million of both company and
government money testing the recovery system, since that is the only
part of the launch vehicle that is not conventional. Caluori thinks
Boeing's chances of winning the Space Lifter competition "are
excellent. I think we've got a really good reception from our customers
on this." In proof, Caluori sited Department of Defense's continued
willingness to fund Boeing's SSME recovery tests during massive overall
cuts in military spending.

In the absence of a new Space Lifter launch vehicle,
Rocketdyne sees an on-going market for refurbished Space Shuttle
engines. Whether the United States develops a new launch vehicle, or
continues with the existing fleet, "The SSME is going to play a major
role," said McMillion. "The Space Transportation System will certainly
keep flying for a long time. And then if something does happen in these
other arenas, the SSME is certain to be a player there."

Something had better happen in another arena, according to
Boeing's Caluori. "We think our space program is in a dead spiral, we
need [a new launch vehicle] as soon as possible, and we certainly need
it to maintain any kind of international competitiveness. We're losing
this [commercial launch vehicle] battle left and right. With the
Russians and Chinese coming on as strong as they are, how [long] are we
going to sit back" and wait for some new technology before getting
started on a real, working vehicle. "We don't have the money, and we
don't have the time" to wait for exotic technology.

Once more sounding more like the salesman than the customer,
Boeing's Caluori was barely able to contain his anger when discussing
the way the Space Shuttle Main Engine is treated in the press and on
Capital Hill. Because of its association with the Shuttle, the SSME "is
perceived in a terrible light. We have invested maybe five billion
dollars in this engine. It is a remarkable, incredible, absolutely
world-class engine. And we are willing to walk past it because some
people have some poor perceptions of it.